In modern x-ray diagnostics, computer tomographs are increasingly employed in which the patient is scanned in a transverse layer by means of a measuring arrangement comprised of an x-ray source and a radiation receiver. From the measured values, delivered by the radiation receiver, which measured values correspond to the attenuation of the x-radiation in the patient, a computer calculates an image of the scanned transverse layer. An important field of application of a computer tomograph is the examination of the cranium; for example, for the detection of tumors. If a tumor is ascertained in the x-ray image, it is necessary to remove a tissue specimen from the tumor tissue. The problem which arises here is to guide a biopsy needle in a directed fashion through a bone gap in the skull in such a fashion that said needle precisely reaches the location of the tissue at which a tissue specimen is to be removed.